Persistence Frequency: Specifies
how often the session state is stored. Applicable only if the Persistence
Type is ha or replicated.
Allowed values are:

web-method - The session state is stored
at the end of each web request prior to sending a response back to the client.
This mode provides the best guarantee that the session state is fully updated
in case of failure. This is the default.

time-based - The session state is stored
in the background at the frequency set by the reapIntervalSeconds store
property. This mode provides does not guarantee that session state is fully
updated. However, it can provide a significant performance improvement because
the state is not stored after each request.

Persistence Scope : Specifies how
much of the session object and how often session state is stored. Applicable
only if the Persistence Type is ha or replicated. Allowed values are as follows:

session - The entire session state is stored
every time. This mode provides the best guarantee that your session data is
correctly stored for any distributable web application. This is the default.

modified-session - The entire session state
is stored if it has been modified. A session is considered to have been modified
if HttpSession.setAttribute() or HttpSession.removeAttribute() was called. You must guarantee that setAttribute() is
called every time an attribute is changed. This is not a Java EE specification
requirement, but it is required for this mode to work properly.

modified-attribute - Only modified session
attributes are stored. For this mode to work properly, you must follow a few
guidelines:

Call setAttribute() every time the session
state is modified.

Make sure there are no cross-references between attributes.
The object graph under each distinct attribute key is serialized and stored
separately. If there are any object cross references between the objects under
each separate key, they are not serialized and deserialized correctly.

Distribute the session state across multiple attributes, or
at least between a read-only attribute and a modifiable attribute.